It is well known that non-relativistic or semi-relativistic constituent quark models can describe a large number of the meson and baryon properties with surprising accuracy. In this work, we study Killingbeck potential using th WKB approximation in the constituent quark model. The implicit approximate solutions for the energy eigenvalues, together with analytical expressions for the root-mean-square radii and the probability densities at the origin, of the Schr"{o}dinger equation are obtained by using appropriate quantization conditions. Finally, we discuss applications to double-heavy meson spectroscopy within the Killingbeck potential.